The term “HVAC” (Heating, Ventilating, and Air Conditioning) is applied to equipment that provides conditioned air to a building space by heating and/or cooling filtered outdoor air in order to maintain desired temperature and humidity conditions within the building space. Depending on outdoor air conditions, on the equipment within the building, and on human comfort requirements, the filtered outdoor air is heated and/or cooled before it is distributed into the building space. As outdoor air is drawn into the building, indoor air is exhausted or allowed to escape from the building, thus removing air contaminants from the building.
Generally, there are two types of HVAC systems of the kind described above, constant air volume systems and variable air volume systems. Constant air volume systems, as their name suggests, typically deliver a constant airflow to each building space. Changes in space temperatures are made by heating and/or cooling the air or by switching an air-handling unit on and off. Accordingly, constant air volume systems do not modulate the volume of air supplied to the building spaces. These constant air volume systems often operate with a fixed minimum percentage of outdoor air or with an air economizer.
In constant air volume systems, the volume of outside air supplied to the building space is constant, irrespective of occupancy. Therefore, a higher than expected occupancy results in poor indoor air quality, and a lower than expected occupancy leads to a waste of energy.
Variable air volume systems maintain the comfort level of a building space by varying the amount of heated and/or cooled air delivered to the building space, and by controlling the air temperature as required. The volume of the outside air supplied to the building space is changed in order to maintain the temperature constant. Alternatively, the speed of the blower fan is controlled to maintain a constant temperature.
However, variable air volume systems have limitations. For example, the relationship between a change in temperature of the building space and the fresh air required to meet indoor air quality standards is not a linear one. This non-linearity presents unnecessary obstacles to the system designer and often leads to poor indoor air quality. Also, temperature is often the primary control factor. Therefore, compromises on air quality are often made.
In current HVAC systems, the ventilation rate for a building depends on its design occupancy, rather than on the actual occupancy in the building. Selecting the ventilation rate according to the design occupancy of the building means that the indoor air quality is poor when occupancy exceeds the design occupancy, that the HVAC system consumes excessive power during periods when occupancy is substantially lower than the design occupancy, and that the air quality standards often are not met due to the non-linearity between the desired temperature and air quality. This latter problem occurs because indoor air quality maintained by a variable air volume system varies with occupancy in order to maintain a set temperature. Thus, even though care is taken in the design of variable air volume systems, 100% air quality control is not ensured.
In the case of constant air volume systems, care is taken primarily to maintain the temperature, and not much importance is given to indoor air quality. Similarly, in the case of variable air volume systems, the volume of air is changed to maintain the set point temperature, but less care is given to air quality. Such systems do not adequately maintain indoor air quality, If people are working in a place where indoor air quality is poor, their working efficiency tends to go down, and they may suffer long-term health problems.
Demand controlled ventilation systems that rely on CO2 sensors as inputs are also known. In such systems, the amount of outdoor (fresh) air brought into a building space is dependent on the level of CO2 and the CO2 level is, at least theoretically, dependent on the number of occupants. Thus, as the CO2 level increases, more outdoor air is brought into the building space, and as the CO2 level decreases, less outdoor air is brought into the building space. However, such systems have several problems. For example, CO2 sensors are costly. Also, the data obtained from the sensors vary rapidly. For example, CO2 sensors are too sensitive to the proximity of people, and the values of CO2 as measured by CO2 sensors depends on their positioning within the building in which they are used. Also, sensitivity varies between sensors, and with aging of the sensors.
The present invention overcomes one or more of these or other problems of prior ventilation control systems.